Vitamin E, an essential fat-soluble vitamin, encompasses eight naturally occurring compounds in two classes. The first class, tocopherols, have four members designated alpha, beta, gamma and delta. The two major forms, .alpha.-tocopherol and .gamma.-tocopherol, differ structurally only by a methyl group substitution at the 5-position. The second class, tocotrienols, are molecules related to the tocopherols and also consist of four members designated alpha, beta, gamma and delta. The tocotrienol structure differs from the tocopherols by possessing three double bonds in their side chain rather than being saturated.
One of the important chemical features of the tocopherols is that they are redox agents which act under certain circumstances as antioxidants. In acting as an antioxidant, tocopherols presumably prevent the formation of toxic oxidation products, such as perioxidation products formed from unsaturated fatty acids. Early on, investigators attributed most if not all of the biological activity of the tocopherols to their ability to act as antioxidants. More recently, however, other biological activities have been associated with tocopherols including the modulation of signal transduction, modulation of phospholipid metabolism, inhibition of protein kinase C, inhibition of phospholipase A and inhibition of prostaglandin production. (Meydani and Mosen, The Lancet 345(8943):170-175 (1995)).
Further, it has recently been discovered that individual members in the class of tocopherols may exhibit different biological properties from one another despite their structural similarity. Some investigators, for example, believe that .gamma.-tocopherol, unlike .alpha.-tocopherol, acts in vivo as a trap for membrane-soluble electrophilic nitrogen oxides and other electrophilic mutagens. (Christen et al Proc. Natl. Acad. Sci. 94: 3217-3222 (1997)). In contrast, others report that .alpha.-tocopherol is a more powerful antioxidant and has ten times the biological activity of .gamma.-tocopherol. (Meydani and Mosen, The Lancet 345(8943):170-175 (1995)). Alpha-tocopherol is also thought to be retained in the body longer than .gamma.-tocopherol and has been shown to preferentially reincorporate into nascent very low-density lipoproteins (LDL). (Christen et al. Proc. Natl. Acad Sci. 94: 3217-3222 (1997)). At present, an understanding of the differences in biological activity of the four tocopherols and their effect on the body is in its infancy.
Alpha tocopherol is largely considered the most important member of the class of tocopherols because it constitutes about 90% of the tocopherols found in animal tissues and displays the greatest biological activity in the commonly used bioassay systems. In consequence, vitamin E supplements are almost exclusively made of .alpha.-tocopherol and little investigation into the efficacy of supplementation with .gamma.-tocopherol has been conducted.
The therapeutic benefits of vitamin E supplementation remains a subject of considerable debate. Several studies have proposed that vitamin E supplementation may prevent a plethora of ills but many of these studies fail to provide causal connections between vitamer supplementation and therapeutic benefit; they merely indicate that a high dietary or plasma concentration and supplemental intake of vitamin E is associated with a reduced risk of disease. In fact, some studies have failed to demonstrate that tocopherol supplementation provides any protection from disease. (Meydani and Mosen, The Lancet 345(8943): 170-175 (1995) and (Christen et al. Proc. Natl. Acad. Sci. 94: 3217-3222 (1997)). A reliable method to treat and prevent diseases associated with oxidative stress and vitamin E deficiency is highly desirable.